WASHINGTON, April (Xinhua) -- The U.S. space agency NASA announced on Wednesday that the first unmanned test flight of Boeing's Starliner spacecraft would be delayed from April to August, and it was only "a working date" yet to be confirmed.

NASA cited limited launch windows in April and May and a U.S. Air Force launch in June on the launchpad by which Starliner is expected to be lifted off as reasons for the adjustment.

The spacecraft's flight with astronauts on board is now targeted for late 2019, again to be confirmed later, according to NASA. The schedule for this manned flight is previously targeted at August.

NASA also said the duration of Starliner's first manned test flight to the International Space Station will be extended but did not say how long the crew would stay on the space lab.

"NASA's assessment of extending the mission was found to be technically achievable without compromising the safety of the crew," said Phil McAlister, director of the commercial spaceflight division at NASA Headquarters. The agency did not say how many months the crew would stay on station.

The extended duration test flight offers NASA the opportunity to complete additional microgravity research, maintenance, and other activities while Boeing's Starliner is docked to station, according to NASA.

NASA also contracted Boeing to develop its new heavy-lift rocket Space Launch System (SLS), critical to its plan to land astronauts on the Moon by 2024. The SLS development is again delayed, casting shadow on the country's "aggressive" plan for deep space exploration.